X-ray mammography is the primary clinical screening tool for breast cancer. Over 15 million mammograms were performed in 1995 and over 25 million are expected to be performed by 2000. However mammography suffers from a high false positive rate. Currently, biopsies are performed following a positive mammogram to determine whether a suspicious lesion is cancerous or benign. Of the approximately 800,00 biopsies performed in 1995, roughly 600,000 were conducted on benign lesions. In addition to the expense involved, biopsy is a stressful procedure for the patient and the scarring left by the biopsy makes subsequent mammograms more difficult to interpret. Additionally, about 15-25% of all women have breast tissue that results in indeterminate mammograms. Dense tissue and scarring from prior surgery have x-ray densities similar to breast lesions, resulting in low contrast mammograms that are difficult to interpret.
Scintimammography has been shown to be able to complement mammography by imaging the metabolic activity of cancerous lesions while ignoring benign lesions and healthy tissue. In studies conducted over the past five years involving 600 women, in connection with the approval process of the DuPont Merck Pharmaceutical Company imaging agent Miraluma.TM., it was concluded that scintimammography is useful in differentiating cancerous and benign lesions. However, the studies also concluded that current large field-of-view gamma cameras cannot reliably image breast lesions smaller than 1.2-1.5 cm. In addition, the large size of these cameras limits their use to the lateral (side) views and does not allow for imaging the breast from other desirable viewing angles, and lesions in the chest wall are very difficult to detect.
U.S. Pat. No. 5,753,917 to Engdahl issued May 19, 1998 describes a multilayer gamma ray discrimination device comprising a pair of scintillator layers mounted in front of an array of photomultiplier tubes for purposes of discriminating between two types of gamma rays. The photomultipliers are neither position sensitive, nor are they or their capabilities described as high resolution. Each individual photomultiplier is apparently capable of individual detection, however collectively they possess no positioning capability and for this reason would not provide the resolution necessary to produce the resolution and positioning capability required of cameras of the type described and claimed herein.
Through the use of novel gamma detector technology, the camera of the present invention images at a higher resolution than can be achieved with conventional gamma cameras, allowing smaller tumors to be seen. The smaller physical size of the camera of the present invention allows close imaging in both lateral and cranial-caudal (top) views.